1. Field of the Invention
The present invention relates to optics. More specifically, the present invention relates to outcouplers for master oscillator power amplifier (MOPA) systems.
2. Description of the Related Art
A High Energy Laser (HEL), because of its rapid time of flight, pointing agility, precision, lack of collateral damage effects, and lack of traceable residue, is an effective weapon against a broad range of military targets. One currently advantageous high energy laser is a PCMOPA (Phase Conjugate Master Oscillator Power Amplifier). The PCMOPA is described and claimed in U.S. patent application Ser. No. 10/340,277 entitled Self-Adjusting Interferometric Outcoupler And Method, filed Jan. 10, 2003 by A. A. Betin et al. the teachings of which are hereby incorporated herein by reference. PCMOPAs currently require a device called an ‘outcoupler’ to output a high power laser beam. This ‘outcoupler’ must output the beam without affecting a reference beam output by the PCMOPA.
The basic phase conjugate (PC) MOPA architecture uses a small master oscillator, which delivers a low-power single-mode reference beam through an optical input/output coupler element (outcoupler) to the output end of a high power amplifier beamline. The beam is then amplified to medium power, picking up thermal lensing and wedging aberrations and is depolarized due to thermal stress birefringence. At this point the beam enters a phase conjugate mirror, which reverses the wavefront of the beam. The reflected, phase conjugate beam then makes a return pass through the aberrated amplifier beamline and the original wavefront is restored. A high power, high beam quality beam is delivered via the outcoupler.
One of the most critical components in this PC MOPA laser architecture is the outcoupler, which is responsible for inserting the low power master oscillator beam into the amplifier beamline and extracting the amplified beam from the beamline in a separate path. Ideally, the outcoupler would insert the oscillator beam with zero loss, extract the amplified beam with zero feedback into the oscillator, and generate no distortions that cannot be corrected by the phase conjugate mirror (PCM). Several outcoupler schemes have been developed and used with the PC MOPA architecture.
One approach involved the use of a simple beamsplitter as an outcoupler. However, with this scheme, a substantial amout of power is returned to the master oscillator and has a tendency to interfere with the reference beam generated thereby. Other approaches have involved a separation of the high energy and the reference beams. One approach, for example, uses a Faraday rotator to rotate the polarization of one of the beams. Unfortunately, Faraday rotators can not currently be scaled to the high power levels needed for current and future applications.
Another approach involves the use of a frequency/wavelength discrimination scheme. This approach typically involves the use of Stimulated Brillouin Scattering (SBS) based phase conjugator and an interferometer to create a constructive interference in one direction and a destructive interference in the other. However, this approach is limited in application and may not be used with the advantageous loop phase conjugate mirror (LPCM) based system.
Another approach was described in U.S. patent application Ser. No. 10/340,277 entitled Self-Adjusting Interferometric Outcoupler And Method, filed Jan. 10, 2003 by A. A. Betin et al. the teachings of which are hereby incorporated herein by reference. This approach proposes the use of a number of Mach-Zender interferometers to determine and control the frequencies of the master oscillator and the loop PCM to achieve the necessary frequency separation. Unfortunately, the interferometers are not easy to use and may be difficult to use with aberrated beams.
Hence, there is a need in the art for a more robust, scalable, element to serve as an outcoupler capable or working with a variety of conjugators including a loop type conjugator.